Reciprocal effects of programmed cell death on fitness in unicellular endosymbiotic <i>Chlorella</i> and its ciliate host

Programmed cell death (PCD), the genetically controlled active cellular suicide mechanism in multicellular organisms, also exists in unicellular organisms. However, explaining the evolution of PCD by natural selection in these organisms remains a challenge. PCD likely emerged during early endosymbiotic events as an initial antagonistic adaptation, enabling unicellular parasitic proto-endosymbionts to exploit their hosts, for example, by triggering host death in response to nutrient depletion or releasing offspring. Over time, during endosymbiont domestication and, as proposed, through horizontal gene transfer from endosymbionts to the host, PCD evolved in the host, providing benefits to both the host and the endosymbionts. However, the underlying assumption of this hypothesis, that PCD benefits and non-PCD (necrosis) harms the endosymbionts and/or the host, remains untested. Here, we investigated the fitness consequences of heat-shock-induced PCD in the endosymbiotic chlorophyte Chlorella variabilis and its facultative symbiotic ciliate host Paramecium bursaria, the non-symbiotic C. sorokiniana, and the predatory host P. duboscqui. Heat shock triggered PCD in C. variabilis and the two ciliate species, causing significant fitness consequences. The supernatant from C. variabilis PCD enhanced the growth of its own clones and endosymbiotic host while inhibiting the growth of the predatory host. The supernatants from necrotic C. variabilis reduced growth of both Chlorella and Paramecium. Similarly, PCD in the symbiotic Paramecium host benefited Chlorella, whereas PCD and necrosis in the predatory Paramecium host were detrimental. These results expand the understanding of unicellular PCD, highlighting its dual role in benefiting clonal populations and their specific endosymbiotic partners, thereby affecting endosymbiosis evolution.